1. Field of the Invention
The present invention relates to an antenna apparatus suitable for a small portable radio unit, in particular, to an antenna apparatus that operates as a mono-pole antenna in its extended state and as a helical antenna in its retracted state.
2. Description of the Related Art
Portable radio units such as portable telephone terminals and PHS (Personal Handyphone System) terminals have been become common. These portable radio units have been developed so as to improve the performance, user interface, and portability. To satisfy such requirements, high density LSI devices and high power batteries are used so as to reduce the size and weight thereof.
Such portable radio units each have a telescopic antenna that can be freely extended and retracted. In an early-staged portable radio unit, before the user uses it, he or she should extend the antenna. The antenna is a simple whip antenna that operates as for example a .lambda./4 (.lambda. is wavelength) mono-pole antenna in the extended state. However, when a portable radio unit is used, the antenna is not always extended. Some users may communicate with their parties in the state that the antennas are retracted. In addition, when the portable radio units are not used, their antennas are always retracted. Thus, it is necessary to consider the dynamic characteristics of the antennas in the retracted state. In the case of a simple whip antenna, when the antenna is retracted, since the antenna is disposed in the vicinity of a grounded conductor, the input impedance increases. Thus, since the impedances are not matched, a sufficient gain cannot be obtained.
To improve the gain of an antenna in the retracted state, a top loading type antenna of which a helical portion is connected to a top portion of the whip antenna is used. In the case of this antenna, when the antenna is extended, the combined portion of the whip antenna and the helical portion operates as a top loading type mono-pole antenna. When the antenna is retracted, only the top helical portion operates as a helical antenna. Thus, in the state that the antenna is retracted, the gain thereof is improved.
However, this antenna has a mono-pole antenna portion that does not radiate radio waves in the retracted state. This portion operates as an open stub that adversely affects the input impedance of the antenna. This portion delicately disturbs the matching state depending on the distance with a circuit board in the portable radio unit. Thus, the operating characters of such an antenna are not high. In addition, when the portable radio unit is not properly shielded, the mono-pole antenna that is retracted collects signals. Alternatively, signals enter the inside of the portable radio unit.
To solve such a problem, an antenna that has a mono-pole antenna and a helical antenna that operate depending on whether the antenna is extended or retracted has been developed. In this antenna, since the mono-pole antenna and the helical antenna separately operate, they do not interfere with each other. Thus, a sufficient gain can be obtained regardless of whether the antenna is extended or retracted.
FIGS. 1A and 1B are sectional views showing the above-described antenna. In FIGS. 1A and 1B, reference numeral 101 is a case. The case 101 is composed of a non-metal material. The case 101 houses a circuit board 102 necessary for a portable radio unit. The circuit board 102 includes an RF transmitting/receiving circuit 103.
The case 101 has an antenna mounting hole 104. A case mounting metal fastener 105 fits with the antenna mounting hole 104. The case mounting metal fastener 105 is electrically connected to an antenna matching circuit 107 through an antenna feeder spring 106. The antenna matching circuit 107 is disposed so as to match the impedances of the RF transmitting/receiving circuit 103 and the mono-pole antenna or the helical antenna.
Reference numeral 112 is an antenna cover composed of an insulator. An upper metal fastener 113 fits with an upper portion of the antenna cover 112. A lower metal fastener 114 fits with a lower portion of the antenna cover 112. A helical antenna portion 111 is disposed between the upper metal fastener 113 and the lower metal fastener 114. An upper portion of the helical antenna portion 111 is electrically connected to the upper metal fastener 113. A lower portion of the helical antenna portion 111 is electrically connected to the lower metal fastener 114.
A hole 115 is formed at an upper center portion of the antenna cover 112. Holes 116 and 117 are formed at a center portion of the upper metal fastener 113 and a center portion of the lower metal fastener 114, respectively. The upper hole 115 of the antenna cover 112, the hole 116 of the upper metal fastener 113, and the hole 117 of the lower metal fastener 114 form a through-hole of the case 101. A mono-pole antenna portion 121 is slidably inserted into the through-hole. An antenna cover 123 composed of an insulator is disposed at an upper portion of the mono-pole antenna portion 121. A top portion 123A of the antenna cover 123 operates as an antenna retracting stopper and an antenna extending knob. An antenna extending stopper 124 composed of a metal material is disposed at a lower portion of the mono-pole antenna 123.
As shown in FIG. 1A, when the antenna is retracted, the mono-pole antenna portion 121 is held in the unit. At this point, the insulator antenna cover 123 disposed on the mono-pole antenna portion 121 contacts the upper metal fastener 113 and the lower metal fastener 114. Thus, only the helical antenna portion 111 operates as for example .lambda./4 helical antenna. Since the antenna cover 123 is composed of an insulator, the RF transmitting/receiving circuit 103 is insulated from the mono-pole antenna portion 121. Thus, the mono-pole antenna portion 121 does not operate.
As shown in FIG. 1B, when the antenna is extended, the mono-pole antenna portion 121 is protruded from the unit. In the state that the antenna is extended, the antenna extending stopper 124 fits with the upper metal fastener 113 and the lower metal fastener 114. Thus, the mono-pole antenna portion 121 is kept in the extended state. Since the antenna extending stopper 124 is composed of a conductor, when it contacts the upper metal fastener 113 and the lower metal fastener 114, both ends of the helical antenna portion 111 are short-circuited. Thus, the helical antenna portion 111 does not operate. The RF transmitting/receiving circuit 103 is connected to the antenna extending stopper 124 through the antenna matching circuit 107, the antenna feeder spring 106, the case mounting metal fastener 105, and the lower metal fastener 114. In addition, the antenna extending stopper 124 is electrically connected to the mono-pole antenna portion 121. Thus, the combined portion of the mono-pole antenna portion 121 and the antenna extending stopper 124 operates as a .lambda./4 mono-pole antenna.
Thus, in the conventional antenna, since different antenna portions independently operate depending on whether the antenna is extended or retracted, good antenna characteristics can be obtained regardless of whether the antenna is retracted or extended.
However, it is said that when the electrical length of a helical antenna is .lambda./4, it has the best characteristics. On the other hand, due to an influence of the head of the user of the unit, when the electrical length of the mono-pole antenna is 3.lambda./8 or .lambda./2, it has the best characteristics. Thus, it is possible to design an antenna having a helical antenna with a length of .lambda./4 and a mono-pole antenna with a length of 3.lambda./8 or .lambda./2.
However, when the electrical lengths of the antenna portions differ from each other, the structure of the matching circuit should be changed. In the above-described related art reference, when the antenna is retracted, the helical antenna operates. On the other hand, when the antenna is extended, the mono-pole antenna operates. In the related art reference, a common antenna matching circuit is disposed for both the helical antenna and the mono-pole antenna. Since the common antenna matching circuit is disposed, it is difficult to structure an antenna having a helical antenna and a mono-pole antenna with different electrical lengths.